This invention relates to a method of fabricating a circuit board.
It is at present known to manufacture a multilayer circuit board from multiple composite sheets each comprising a substrate of dielectric material having a foil of conductive material, such as copper, adhered to one surface thereof. The copper foil of the first composite sheet is patterned so as to define a first layer of conductor runs, and the second sheet is bonded to the first sheet, with its copper foil away from the first sheet. The copper foil of the second sheet is patterned to define a second layer of conductor runs. Generally, it is desired that there be connections between conductor runs of the first layer and conductor runs of the second layer. This is accomplished by drilling holes through the the first and second sheets before the copper foil of the second sheet is patterned, and electrolessly plating copper into the holes. If the holes are properly positioned, the through-hole plating establishes a conductive via between a conductor run of the second layer and a conductor run of the first layer.
Of course, this same procedure may be applied to circuit boards having three or more layers of conductor runs, and in such case may be used to interconnect runs of more than two layers.
When multilayer circuit boards were first introduced, the through-hole metal was provided not by plating but by insertion of a metal eyelet into the hole. At that time, it was felt that it was necessary for there to be contact with the eyelet around its entire periphery in order for there to be an adequate electrical connection between the eyelet and a conductor run. Accordingly, at the locations at which through holes were to be made, the conductor runs of the first layer were formed with enlarged pads. For example, a conductor run might be 20 mil wide (one mil is 0.001 inch, or 0.0254 mm) and terminate in a pad that is 40 mil in diameter. A hole that is perhaps 30 mil in diameter is drilled through the pad.
Modern circuit board fabrication techniques allow conductor runs that are six or eight mil wide. However, it has not been possible to reduce the size of the pads proportionately, because the pads must be larger than the through hole, and the through holes cannot be less than about 30 mil diameter, since if they were any smaller the drill bit would be too fragile.
As the width of conductor runs has decreased, the proportion of a conductor layer that is occupied by pads has increased. In a typical circuit board, pads occupy about 30 percent of the area of each conductor layer. Moreover, where the distance between through holes is limited, for example because the through holes serve not only to accommodate a via but also as mounting holes for a component, the pads limit the number of conductor runs that can pass between the holes.